Arsenite extrusion pumps have been identified in bacteria, mice and humans. A hydrophilic transporter belonging to the ATPase superfamily, having no transmembrane domain and having two potential adenylate-binding sites has been studied in arsenite-resistant bacteria (Chen, C. M. et al. (1986) J. Biol. Chem. 261:15030-15038). Lower molecular weight homologs have been found in mice and humans where the different domains may be encoded by two different genes (Kurdi-Haidar, B. et al. (1996) Genomics 36:486-491).
Plants require certain essential elements for completing their life cycle. Energy from sunlight and availability of the essential elements allows plants to synthesize the compounds necessary for their normal growth. One of these essential elements is zinc which is absorbed from the soil, is required for the activity of many enzymes and is required for chloroplast biosynthesis in some plants. Zinc deficiency results in small plants with distorted leaves; in some species such as corn, sorghum and beans the leaves show necrotic spots and chlorosis. Excess zinc is also detrimental for plants. To prevent zinc toxic buildup in the cytosol, plants may sequester it in the vacuole. The zinc transporter ZnT-2 protects cells from zinc toxicity by facilitating vacuolar zinc transport into an endosomal/lysosomal compartment (Palmiter, R. D. et al. (1996) EMBO J. 15:1784-1791).